Soil Pollution

Photo by: Nejron Photo

Soil pollution comprises the pollution of soils with materials, mostly
chemicals, that are out of place or are present at concentrations higher
than normal which may have adverse effects on humans or other organisms.
It is difficult to define soil pollution exactly because different
opinions exist on how to characterize a pollutant; while some consider the
use of pesticides acceptable if their effect does not exceed the intended
result, others do not consider any use of pesticides or even chemical
fertilizers acceptable. However, soil pollution is also caused by means
other than the direct addition of xenobiotic (man-made) chemicals such as
agricultural runoff waters, industrial waste materials, acidic
precipitates, and radioactive fallout.

Both organic (those that contain carbon) and inorganic (those that
don't) contaminants are important in soil. The most prominent
chemical groups of organic contaminants are fuel hydrocarbons, polynuclear
aromatic hydrocarbons (
PAHs
), polychlorinated biphenyls (
PCBs
), chlorinated aromatic compounds, detergents, and pesticides. Inorganic
species include nitrates, phosphates, and heavy metals such as cadmium,
chromium and lead; inorganic acids; and
radionuclides
(radioactive substances). Among the sources of these contaminants are
agricultural runoffs, acidic precipitates, industrial waste materials, and
radioactive fallout.

An area of Karabache, Russia, where soil has been poisoned by high
concentrations of lead, arsenic, nickel, cobalt, and cadmium. (

Soil pollution can lead to water pollution if toxic chemicals leach into
groundwater, or if contaminated runoff reaches streams, lakes, or oceans.
Soil also naturally contributes to air pollution by releasing volatile
compounds into the atmosphere. Nitrogen escapes through ammonia
volatilization and
denitrification
. The decomposition of organic materials in soil can release sulfur
dioxide and other sulfur compounds, causing acid rain. Heavy metals and
other potentially toxic elements are the most serious soil pollutants in
sewage. Sewage sludge contains heavy metals and, if applied repeatedly or
in large amounts, the treated soil may accumulate heavy metals and
consequently become unable to even support plant life.

In addition, chemicals that are not water soluble contaminate plants that
grow on polluted soils, and they also tend to accumulate increasingly
toward the top of the food chain. The banning of the pesticide DDT in the
United States resulted from its tendency to become more and more
concentrated as it moved from soil to worms or fish, and then to birds and
their eggs. This occurred as creatures higher on the food chain ingested
animals that were already contaminated with the pesticide from eating
plants and other lower animals. Lake Michigan, as an example, has 2 parts
per trillion (ppt) of DDT in the water, 14 parts per billion (ppb) in the
bottom mud, 410 ppb in amphipods (tiny water fleas and similar creatures),
3 to 6 parts per million (ppm) in fish such as coho salmon and lake trout,
and as much as 99 ppm in herring gulls at the top of the food chain.

The ever-increasing pollution of the environment has been one of the
greatest concerns for science and the general public in the last fifty
years. The rapid industrialization of agriculture, expansion of the
chemical industry, and the need to generate cheap forms of energy has
caused the continuous release of man-made organic chemicals into natural
ecosystems. Consequently, the atmosphere, bodies of water, and many soil
environments have become polluted by a large variety of toxic compounds.
Many of these compounds at high concentrations or following prolonged
exposure have the potential to produce adverse effects in humans and other
organisms: These include the danger of acute toxicity, mutagenesis
(genetic changes), carcinogenesis, and teratogenesis (birth defects) for
humans and other organisms. Some of these man-made toxic compounds are
also resistant to physical, chemical, or biological degradation and thus
represent an environmental burden of considerable magnitude.

Numerous attempts are being made to decontaminate polluted soils,
including an array of both
in situ
(on-site, in the soil) and off-site (removal of contaminated soil for
treatment) techniques. None of these is ideal for remediating contaminated
soils, and often, more than one of the techniques may be necessary to
optimize the cleanup effort.

The most common decontamination method for polluted soils is to remove the
soil and deposit it in landfills or to incinerate it. These methods,
however, often exchange one problem for another: landfilling merely
confines the polluted soil while doing little to decontaminate it, and
incineration removes toxic organic chemicals from the soil, but
subsequently releases them into the air, in the process causing air
pollution.

For the removal and recovery of heavy metals various soil washing
techniques have been developed including physical methods, such as
attrition
scrubbing and wet-screening, and chemical methods consisting of
treatments with organic and inorganic acids, bases, salts and chelating
agents. For example, chemicals used to extract radionuclides and toxic
metals include hydrochloric, nitric, phosphoric and citric acids, sodium
carbonate and sodium hydroxide and the chelating agents EDTA and DTPA. The
problem with these methods, however, is again that they generate secondary
waste products that may require additional hazardous waste treatments.

In contrast to the previously described methods,
in situ
methods are used directly at the contamination site. In this case, soil
does not need to be excavated, and therefore the chance of causing further
environmental harm is minimized.
In situ
biodegradation involves the enhancement of naturally occurring
microorganisms by artificially stimulating their numbers and activity. The
microorganisms then assist in degrading the soil contaminants. A number of
environmental, chemical, and management factors affect the biodegradation
of soil pollutants, including moisture content, pH, temperature, the
microbial community that is present, and the availability of nutrients.
Biodegradation is facilitated by aerobic soil conditions and soil pH in
the neutral range (between pH 5.5 to 8.0), with an optimum reading
occurring at approximately pH 7, and a temperature in the range of 20 to
30°C. These physical parameters can be influenced, thereby
promoting the microorganisms' ability to degrade chemical
contaminants. Of all the decontamination methods bioremediation appears to
be the least damaging and most environmentally acceptable technique.

PHYTOREMEDIATION

Plants can absorb, accumulate and in some cases break down pollutants such
as heavy metals, pesticides, and explosives in soil and groundwater. Now
the United States Department of Agriculture and the Department of Energy
are conducting pilot studies to investigate whether plants can also remove
radionuclides from soil. By adding soil amendments such as ammonium
compounds, the pigweed plant, Amaranthus retroflexus, will absorb
cesium-137 that contaminates soil at some DOE sites due to aboveground
nuclear testing during the Cold War era.

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